This application claims priority on Finnish Application No. 20001307, filed May 31, 2000, the disclosure of which is incorporated by reference herein.
Not applicable.
The invention relates to dryer groups in a dryer section of a paper or board machine, and to dryer sections of paper or board machines.
As known from the prior art, in multi-cylinder dryers of a paper machine, twin-wire draw and/or single-wire draw in/are employed, In twin-wire draw, the groups of drying cylinders comprise two wires which press the web, one from above and the other one from below, against heated cylinder surfaces. Between the rows of drying cylinders, which are usually horizontal rows, the web has free and unsupported draws, which are susceptible to fluttering, which may cause web breaks, in particular as the web is still relatively moist and, therefore, has a low strength. Therefore, ever increasing use has been made of said single-wire draw in which each group of drying cylinders comprisesxe2x80x94only one drying wire on whose support the web runs through the entire group so that, on the drying cylinders, the drying wire presses the web against the heated cylinder surfaces and, on the reversing cylinders or rolls situated between the drying cylinders, the web remains at the side of the outside curve. Thus, in single-wire draw, the drying cylinders are arranged outside the wire loop, and the reversing cylinders or rolls are arranged inside the loop. One problem with these prior art arrangements is constituted by the cylinders and rolls which are situated outside the wire loop and which get soiled, wherefore it has been necessary to provide them with doctors to keep the surfaces clean.
With increasing speeds of paper machines, the runnability of a paper machine is, of course, also affected by the dryer section, whose length with the prior art multi-cylinder dryers would, at high speeds, also become very long. If it is imagined that a present-day multi-cylinder dryer were used at a web speed of 40 m/s, it would include about 70 drying cylinders, and its length in the machine direction would be about 180 meters. In such a case, the dryer would comprise about 15 separate wire groups and a corresponding number of draws over the group gaps. It is to be assumed that, in a speed range of from 30 to 40 m/s, the runnability of the normal prior art multi-cylinder dryers would not be good because of the open draws, in which connection there may occur web breaks that reduce the efficiency of the paper machine. In a speed range of from 30 to 40 m/s and at higher speeds, the prior art multi-cylinder dryers, then, would also become uneconomical because the investment costs of an excessively long paper machine hall would be high. The dryer sections composed of multi-cylinder dryers are often also very high.
Moreover, in dryer groups of known applications, wires seamed on the machine have to be used because of the closed frame structure. The seam of a wire seamed on the machine may cause a discontinuity in the drying of the web in connection with the drying process. In addition, the seam of the wire seamed on the machine may limit the applied tension of the wire.
Further problems manifested with increased emphasis at high speeds of paper machines, for which problems, at least for all of them, satisfactory solutions have not been found as yet, include the problems of quality related to the requirements of uniformity of the profiles of properties of the paper web both in the machine direction and in the cross direction. Uniformity of the web produced also affects the runnability of the entire paper machine, and it is also an important quality factor of finished paper. Application of high speeds, in particular in wide machines, thus provides ever more difficult problems to be solved, of which problems, the most important ones are runnability and adequate dewatering capacity of the machine at a high speed.
One problem in dryer sections of a paper machine, when conventional dryer sections composed of multi-cylinder dryers are used, is the noise they cause. In addition, when cylinder drying is employed, it is necessary to use steam, which means that the drying cylinders have to be dimensioned to comply with pressure vessel regulations and, furthermore, the hood arranged around the dryer section has to be made to be thermally insulated.
It is known from experience that, if paper is dried one-sidedly, the result is a tendency of curling of the sheet. When paper is dried by means of normal singlewire draw groups from the side of its bottom surface and, if such asymmetric drying is extended substantially over the entire length of the dryer section, the drying occurs such that the side of the bottom surface of the web dries first and, when the drying makes progress, the drying effect is also extended to the side of the top surface of the web. Under these circumstances, the dried paper usually curls and becomes concave, seen from above.
With respect to the prior art related to the invention, reference is made to FI Patent 102623 which discloses a method in a paper machine or equivalent, wherein water is removed from a web or equivalent in a press and, after pressing, the web is dried in at least one dryer group based on impingement drying or equivalent. In this prior art arrangement, the web is guided along a substantially linear path or by using a large radius of curvature and, in the drying stage, after the impingement drying, the web or equivalent is dried in at least one dryer group in which a normal single-wire draw is applied, and the web is passed from the pressing stage to the drying stage as a closed draw and such that the web is constantly supported against at least one support surface. The above-mentioned patent discloses different types of applications for arranging impingement drying units in a dryer section, and with respect to arrangements of this type, reference is also made to the magazine article Markus Oeschle: xe2x80x9cDrying concepts for high demandsxe2x80x9d, Paper ja Puu, Vol. 81, No. 8/1999.
An object of the invention is to provide a dryer section and a dryer group in which the above-noted drawbacks and problems known in the prior art have been eliminated or at least minimized.
In addition, one object of the invention is to provide a dryer section concept that makes it possible to apply shorter and lower dryer sections than heretofore as compared with prior art dryer sections.
A further object of the present invention is to provide a dryer section which allows good runnability and a mainly closed draw of the web to be provided.
A further object of the invention is to enable a dryer section concept to be applied in which good quality of paper and sufficiently undisturbed runnability are achieved.
A further object of the invention is to provide a drying method and a dryer section applying the method in which the web reliably adheres to a drying wire over the entire length of the dryer section such that its cross direction shrinkage can be mainly inhibited, thereby avoiding cross direction inhomogeneities of the web caused by non-uniform cross direction shrinkage profile.
A still further object of the invention is to provide a dryer group and a dryer section having a new type of frame structure enabling compact dryer groups and a compact dryer section to be formed.
A still further object of the invention is to create a dryer group in which a factory-preseamed drying wire loop is threaded in place in the cross direction of the machine.
Thus, an object of the present invention is to offer new solutions to the problems discussed above so that said drawbacks as well as those coming out later in the prior art are chiefly avoided.
In accordance with the invention, the dryer groups of a dryer section are formed of at least one impingement drying unit, by means of which the web is dried directly, i.e. drying blowings are directed directly, not through a wire, at the web and the wire supporting and guiding the run of the web is on the opposite side of the web to the impingement drying unit/units. A frame structure is placed inside the wire loop, and the components and devices arranged inside the wire loop and leading and guiding the run of the wire and the web, advantageously all the components and devices in question, are supported on said frame structure. The frame structure supporting the group is positioned advantageously such that the main direction of the group is inclined with respect to the horizontal direction. Because of the new type of group structure, the dryer section becomes low and short, and the groups are located with respect to one another preferably in an overlapping fashion, whereby the length of the dryer section is further shortened. In accordance with an advantageous application of the invention, the dryer groups in the dryer section are so located that one side of the web is dried first and then the other side, in which connection there arises no one-sided curling tendency in the web.
When a dryer section is formed in accordance with the invention, between the cylinder and the wire there remains no opening gap which would limit speed, but, instead, the dryer groups are so placed that the draw over the group gap is closed.
In the invention, the web which is dried is advantageously supported by means of under-pressure devices, for example, by means of blow or suction boxes and/or suction rolls, in order to further improve runnability, which devices simultaneously assist in preventing web shrinkage.
When impingement drying is applied, no steam is required in the dryer section, in which connection the hood can be built to be simpler because it need not be thermally insulated. Impingement drying is also less noisy than cylinder drying, which means that the sound insulation of the hood is also easier to provide.
Since impingement drying is more effective than multi-cylinder drying, the dryer section becomes shorter when compared with a dryer section formed of multi-cylinder dryers. Furthermore, impingement drying is quicker than multi-cylinder drying in connection with starting of a paper machine.
In the arrangement in accordance with the invention, when compared with conventional multi-cylinder drying, there is less revolving mass, in which connection the problems arising from vibration are minimized. In addition, the design of the foundation is therefore less expensive and the foundation can be built to be lighter-weight.
A guide and tension device assembly is advantageously used in connection with the dryer groups of the dryer section in accordance with the invention, which device is used at the same time for both guiding the run of the wire and for tensioning it to desired tightness.
The dryer groups according to the invention are compact because of their new type of frame structure, in which connection shorter, factory-seamed wires can be used because the frame of the dryer group can be cantilevered for threading the wire loop in place. In addition, the factory-seamed drying wire loop can be considered in practice to be seamless because the fabric structure of the wire loop is uniform and does not cause any discontinuity in the drying of the web. Moreover, a higher wire tension can be used for factory-seamed wires than for wires which are seamed in the machine.
In one advantageous application of the invention, rolls or cylinders are not needed at all outside the wire loop, thereby obviating the need for doctors.
In accordance with the invention, the entire dryer section or at least one dryer group of a part of the dryer section can be formed of dryer groups according to the invention and, when desired, dryer groups according to the invention can thus be used with conventional multi-cylinder dryer groups or other types of dryer groups in the dryer section.
Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.